Drillable Oilfield Tubular Plug

ABSTRACT

A drillable oilfield tubular plug that employs clutch faces, drilling features, anti-rotation features, and composite materials to facilitate quicker drill-out operations after the plugs have been utilized.

REFERENCES TO RELATED APPLICATIONS

None.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to tubular plug tools for petroleum well applications. More particularly, the present invention relates to drillable pressure sealing bridge plugs, ‘frac’ plugs, and packers, which employ composite materials and interlocking features to facilitate quick drill-out operations.

Description of the Related Art

Oil and gas well construction begins with a wellbore drilled into the ground to a predetermined depth. The wellbore is lined with a steel well casing, which is commonly cemented in place within the wellbore. Before the well is placed into production, the casing is perforated at one or more depths to enable well fluids to flow from the formation into the well casing. Various tools may be run down the casing to develop the well and to commence production of hydrocarbon minerals, and to maintain the well over the years. Depending on the petroleum fluid bearing formation into which the well is drilled, various sequences of tools may be used. For example, in the case of a well that has well fluids dispersed into a porous formation, hydraulic fracturing may be employed to facilitate the migration of well fluids into the well casing.

A common requirement in well development and servicing is the need to seal the casing, or other petroleum tubular, against pressure to control movement of fluids between two sides of a particular location in the tubular. For example, in the case of hydraulic fracturing operations, it is necessary to plug the casing below the perforations into which fracturing fluid is pumped in order to assert the requisite pressure needed to fracture the formation. This type of tool is referred to as a “frac” plug. More generically, the term “bridge plug” may be used. Another pertinent term of art is the “packer”, which is a specialized plug used in an annulus between tubulars. Generally, these may simply be referred to as “plugs”. Hydraulic fracturing is just one example, and those skilled in the art are familiar with numerous applications for the use of pressure sealing plugs in oil and gas well operations.

A plug is typically lowered into the tubular using a line, such as an e-line, wire line, or coiled tubing, and is then set into place using a setting tool. There are various setting tools available, but a common mode of operation is a setting tool that engages the plug, and then applies downward force from above the plug while pulling upwardly on a central mandrel of the plug. This action generates high compressive forces within the plug, and this force is used to compress a sealing member of the plug against the interior of the tubular. Such plugs generally incorporate plural slips that bite into the interior circumference of the tubular as the plug is set, and serve to hold the plug in position and also to hold compressive forces on the sealing element to perfect the seal while the pressure operations are undertaken.

Once the pressure operation is completed, the plug generally must be removed so that subsequent operations can be undertaken. A common method to accomplish this is to drill the plug out of the tubular using a well drilling bit such as the common tricone drill bit. The drill bit is lowered to the plug location and then run to grind the plug into small pieces, which may either be pumped to the surface using a liquid, or allowed to fall to the bottom of the wellbore. In the case of a hydraulic fracturing operation, there may be several plugs set in the well casing at incremental depths, all of which must be drilled out. A line crew using specialized equipment is employed to undertake the drilling operations, which carries considerable costs calculated in accordance with the time required to accomplish the job. Thus, it is desirable that the time to drill out each plug be a short as possible. This brings into issue the choice of plug materials, as well as the physical configuration of the plug and its reaction to the drilling operation. Thus it can be appreciated that there is a need in the art for a petroleum tubular pressure plug that enables the pressure sealing requirements, and is readily removable by drilling.

SUMMARY OF THE INVENTION

The need in the art is addressed by the apparatus of the present invention. The present disclosure teaches a drillable plug assembly, which is settable using a tool, for isolating differential pressures in a tubular, including oil and gas well casings and the like. The drillable plug includes a composite material mandrel that has a cylindrical body with a first clutch face formed on an upper end thereof, and a tool thread formed within a central bore thereof that shears at a first predetermined axial force. The mandrel also has a setting thread disposed about the exterior thereof, and an anvil connector disposed at the bottom thereof. A setting sleeve threadably engages the setting thread to attach it about the cylindrical body. These threads shear at a second predetermined axial force applied against them. A composite material anvil is connected to the anvil connector, and has a second clutch face formed on its bottom. A seal assembly, which includes a an elastomeric seal and a slip for sealably and fixedly engaging the tubular upon setting by the tool, is disposed about the cylindrical body between the setting sleeve and the anvil. The first ratchet clutch face and the second ratchet clutch face are configured to cooperatively engage one another and prevent rotation therebetween, to thereby enable plural drillable plug assemblies in the tubular to resist rotation therebetween as they are drilled out of the tubular.

In a specific embodiment of the foregoing apparatus, the composite material is fiber reinforced plastic. In a refinement to this embodiment, the composite material comprises epoxy resin reinforced with filament wound fiber that is selected from glass, aromatic polyamide, and carbon.

In a specific embodiment of the foregoing apparatus, the first clutch face and the second clutch face are configured as interlocking crown gears. In another specific embodiment, the first clutch face and the second clutch face are configured as mating Hirth coupling faces. In another specific embodiment, the first clutch face and the second clutch face are configured as ratchet clutches. In a refinement to this embodiment, the ratchet clutches includes a series of asymmetrical teeth, each having an inclined surface meeting a drive face at an acute angle.

In a specific embodiment of the foregoing apparatus, the second predetermined axial force is greater than the first predetermined axial force, so that the setting sleeve shears from the mandrel prior to the tool shearing from the tool thread. In another specific embodiment, the setting sleeve includes a sleeve thread the shears at the second predetermined axial force.

In a specific embodiment of the foregoing apparatus, the anvil connector is an anvil thread disposed adjacent to the bottom of the cylindrical body, and the anvil threadably engages the anvil thread.

In a specific embodiment of the foregoing apparatus, where the seal assembly slidably engages the tubular body between the setting sleeve and the anvil, a first setting cone and a second setting cone are disposed on either side of the elastomeric seal, and each have a conical portion extending away from the elastomeric seal, and the slip includes a pair of iron slips that have conical interior surfaces that cooperatively engage the pair of setting cones such that they are driven radially into the tubular in response to axial force that exceeds the first predetermined axial force, which compresses the elastomeric seal, and locate and set the drillable plug assembly in the tubular. In a refinement to this embodiment, the setting sleeve and the first and second cone are formed of composite material, and the pair of iron slips are fabricated from ductile iron.

In a specific embodiment of the foregoing apparatus, the anvil is configured with a frustoconical surface extending from the second clutch face, and there are plural helical flutes formed upon the frustoconical surface, to facilitate removable of material within the tubular as the drillable plug assembly is drilled. In a refinement to this embodiment, the second clutch face on the bottom end of the anvil includes and plural inclined surfaces and plural drive surfaces that meet and at plural acute angle vertexes, and the plural helical flutes each intersect one of the plural acute angle vertexes.

In a specific embodiment of the foregoing apparatus, the slip includes an upper planar surface that is engaged by a lower planar surface of the setting sleeve, to thereby transmit axial force from the setting sleeve to the slip as the drillable plug assembly is set by the setting tool. In a refinement to this embodiment, the slip includes plural sections that fracture into slip section pieces as the drillable plug assembly is set by the setting tool, and the lower planar surface of the setting sleeve includes plural recesses formed therein, which are configured to engage the slip section pieces to thereby prevent rotation therebetween as the drillable plug assembly is drilled out of the tubular. In another refinement, the plural recesses are formed as radial grooves in the lower planar surface of the setting sleeve. In yet another refinement, the plural recesses have a width selected to correspond to the size of the slip section pieces to thereby facilitate engagement therebetween.

In a specific embodiment of the foregoing apparatus, a lock is provided for locking the anvil against rotation with respect to the mandrel. In another specific embodiment, the central bore of the anvil extents continuous from the upper end to the lower end, and comprises a ball seat disposed there along for retaining a ball, to thereby provide a passage through which fluids may pass and a means for sealing the central bore using a ball placed against the ball seat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view drawing of a tubular plug according to an illustrative embodiment of the present invention.

FIG. 2 is an exploded view drawing of a tubular plug according to an illustrative embodiment of the present invention.

FIG. 3 is side view drawing of a mandrel for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 4 is a section view drawing of a mandrel for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 5 is a perspective view drawing of a setting sleeve for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 6 is a section view drawing of a setting sleeve for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 7 is a perspective view drawing of a cone for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 8 is a section view drawing of a cone for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 9 is a side view drawing of a slip ring for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 10 is an end view drawing of a slip ring for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 11 is a section view drawing of a slip ring for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 12 is a perspective view drawing of an anvil for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 13 is a side view drawing of an anvil for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 14 is an end view drawing of an anvil for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 15 is a section view drawing of an anvil for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 16 is a side view drawing of an anvil for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 17 is a partial side view of the proximate end of a mandrel for a tubular plug according to an illustrative embodiment of the present invention.

FIG. 18 is a section view drawing of a well casing with plural tubular plugs therein according to an illustrative embodiment of the present invention.

DESCRIPTION OF THE INVENTION

Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.

While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope hereof and additional fields in which the present invention would be of significant utility.

In considering the detailed embodiments of the present invention, it will be observed that the present invention resides primarily in combinations of steps to accomplish various methods or components to form various apparatus and systems. Accordingly, the apparatus and system components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the disclosures contained herein.

In this disclosure, relational terms such as first and second, top and bottom, upper and lower, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

An improved tubular plug designed to be readily drillable is presented in the illustrative embodiments. The dynamics of drilling out a plug are complex. The drill bit is driven against the plug to grind it into small pieces that can be pumped out of the well. This suggests that softer materials may be drilled out more quickly as the drill bit will be able to grind a softer material more quickly. However, the materials must be able to accommodate the compressive and tensile loads at play, as well as the temperatures and chemical exposures involved. For the plug to hold in place after being set by a setting tool, the materials of the slips must be hard enough to bite into the interior of the tubular. The illustrative embodiment presents plugs that use composite materials for the majority of the components, generally fiber reinforced plastics, and iron for the slip elements. The iron slips may employ a case hardened surface to facilitate engagement with the tubular. In one embodiment, ductile iron slips are employed, which have been surface hardened using a ferritic nitrocarburizing process.

At the time the plug is set into the well casing, the slip elements are driven against the interior of the casing using an expansion cone arrangement within the plug. This action fractures a ring-like slip element into plural individual slips, each of which bite into the interior of the tubular. The individual slip elements facilitate drill-out operations by virtue of their reduced size and interaction with other components present in the grinding environment. The composite materials are readily drillable because they are substantially softer than the drill bit. As a single plug is being drilled, it is initially locked into place in the tubular, so it holds still as the drill bit grinds away at it. At some point, the drill bit reaches an upper set of slips, and grinds them away from fixed engagement with tubular, and releases the plug from its fixed location. This action immediately changes the drilling dynamics.

Once the slip disengage from the casing, the plug loses its fixed position, and may begin to rotate with the dill bit, which greatly reduces the effectiveness of the drilling action. The plug may fall within the tubular at this point, coming to rest at whatever may be present in the tubular below. Wherever the plug comes to rest, the drill bit will be lowered to that point to continue drilling of the plug. If the arrangement is such that the plug can rotate in the tubular, drilling time will be increased because the rotating plug has a greatly reduced grinding interface with the drill bit. The present disclosure teaches a drillable plug with several features the resist rotation of the plug during drill-out operations. These features include clutch faces at the upper and lower ends of the plugs, which engage one another, as well as sand and debris in the casing, to resist rotation, and thereby facilitate reduced drill-out time. In the case of plural frac plugs in a single casing, the benefit of upper and lower clutch faces on every plug becomes apparent. When a presently drilled plug falls, it comes to rest on the next lower plug in the casing. The upper clutch face of the lower plug engages the lower clutch face on the upper plug, and thusly resists rotation thereof to enable the drill bit to grind much more quickly.

During drill-out operations of plugs in a tubular, there are other factors that effect efficiency. It is common for sand and other debris to collect above individual plugs as drill-out is undertaken, this debris may interfere with efficient drilling. An illustrative embodiment of the present invention employs a conical anvil at the lower, or distal, end of the plug, which has plural helical flutes formed therein. These flutes, together with the lower clutch face, function like a twist drill bit to drill through the sand and debris quickly until the plug reaches the upper clutch face of the next lower frac plug, or some other tool with which it may engage. In addition, the present disclosure teaches the use of recesses formed in the lower side of a setting sleeve, which is configured to engage the slip pieces to further prevent undesirable rotation of the plug components during drilling. Taken together, the present disclosure teaches a tubular plug that can be drilled very quickly, as evidenced during field test where drill-out times of eight minutes were realized.

Reference is directed to FIG. 1, which is a side view drawing of a tubular plug 2 according to an illustrative embodiment of the present invention. The plug 2 is configured in remarkably compact form, and is primarily fabricated from fiber reinforced plastics, such as epoxy resin and filament wound glass fibers. The sealing action of the plug 2 is accomplished with a seal assembly 10 disposed between an anvil 8 at the lower, or distal, end of the plug 2, and a setting sleeve 6, which is engaged by a mandrel 4 visible at the upper, or proximate, end of the plug 2. The distal face of the anvil 8 presents a clutch face 14, which will be more fully discussed hereinafter. Similarly, the proximate face of the mandrel 4 presents a clutch face 12. The two clutch faces 12, 14 are configured to engage one another and prevent rotation therebetween as least when rotated in the clockwise direction as viewed from the proximate face end. This direction corresponds to the clockwise rotation of down hole tools universally employed due to the use of right-hand threads on petroleum field tubular components and tools. Although, the clutch faces may prevent rotation in either direction. Note that the lower clutch face of a first plug is intended to engage the upper clutch face of a second plug, which would be located below the first plug in the tubular, as is commonly employed in hydraulic fracturing jobs. The clutch faces 12, 14 can also engage other objects encountered in a tubular (not shown) and prevent rotation of the plug 2 as well.

Reference is directed to FIG. 2, which is an exploded view drawing of a tubular plug 2 according to an illustrative embodiment of the present invention. FIG. 2 corresponds to FIG. 1. In FIG. 2, the mandrel 4 is presented, and illustrates a number of structural and functional features. The mandrel 4 is generally cylindrical and acts as the support fixture of the rest of the plug 2 components. At the distal end of the mandrel 4 is an anvil connector 28, which is a male thread on the exterior of the mandrel 4 in this illustrative embodiment. A setting thread 26 is also disposed about the exterior of the mandrel 4, which serves to engage a setting sleeve 6, as will be more fully discussed hereinafter. At the upper, or proximate, end of the mandrel 4 there is an internally threaded central bore 30, which is provided for connection of a setting tool (not shown), as are well known in the art. At the proximate end of the mandrel 4 there is a first clutch face 12 presented, as illustrated.

A setting sleeve 6, in FIG. 2, is slid onto the mandrel 4 from its distal end and threadably engages the setting thread 26 on the exterior of the mandrel 4 body. The threaded engagement between the setting sleeve 6 and the setting thread 26 are designed to shear when axial force is applied therebetween by the setting tool (not shown) at the time the plug 2 is set in a tubular (not shown). In the illustrative embodiment, the axial shear force is approximately 10,000 pounds. As will be appreciated by those skilled in the art, the plug 4 is set in the tubular (not shown) when a setting tool (not shown) applies and upward force on the tool thread 30 while applying a downward force on the upper, or proximate, face of the setting sleeve 6. When this occurs, and the axial force exceeds the shear strength of the threaded engagement with the setting sleeve 6, the plug setting operation is commenced. The setting sleeve 6 is positioned above the seal assembly 10, which is placed above the anvil 8, which is fixed to the anvil connector 28 at the distal end of the mandrel 4. As such, the seal assembly 10, which slidably engages the mandrel 4, is compressed between the anvil 8 and the setting sleeve 6 at the time the plug is set. As further force is applied by the setting tool (not shown) the tool threads 30 also shear, but at a greater level of axial force, which is approximately 28,000 to 30,000 pounds. At this time, the tool becomes disengaged from the plug, and the plug is set by the following actions occurring within the seal assembly 10.

At the time the plug 2 in FIG. 2 is set in the tubular (not shown), the seal assembly 10 is compressed between a pair of cones 18, 20 and a pair of slip rings 22, 24, which are compressed between the anvil 8 and the setting sleeve 6 to at least the degree of force applied to shear the tool thread 30. The compression forces compress elastomeric seal 16, which expands radially to sealably engage the interior wall of the tubular (not shown). At the same time, the slip rings 22, 24 are driven radially outward by the taper of the cones 18, 20, which causes the slip rings 22, 24 to fracture into individual slip elements (discussed further hereinafter), and engage the interior wall of the tubular (not shown). The slip rings 22, 24 have a saw tooth configuration aligned to engage and resist axial spreading of the slips 22, 24, thereby retaining the elastomeric seal 16 under compression and sealably engaged with the tubular (not shown). This is likened to a ratchet action where the slips 18, 20 can be force closer together, but resist moving further apart, as will be appreciated by those skilled in the art. In the illustrative embodiment, all of the components, except slip rings 22, 24, are fabricated from fiber reinforced plastic. This greatly facilitates drilling at the time the plug is drilled-out. It also reduces fabrication costs for the plug 2. As noted hereinbefore, the slips 22, 24 are fabricated from iron, which may be cast or ductile, and which are case hardened to exceed the hardness of the tubular. Other materials could be utilized to fabricate the slips 22, 24.

Reference is directed to FIG. 3 and FIG. 4, which are a side view drawing and a section view drawing, respectively, of a mandrel 4 for a tubular plug according to an illustrative embodiment of the present invention. In the illustrative embodiment, the mandrel is fabricated from an epoxy resin suitable for the thermal and chemical environment of the particular petroleum products at issue, and is reinforced with glass fibers, such as E-glass. The glass fiber is filament wound for high tensile strength. Other fibers, including carbon fibers and aromatic polyamide fibers, can also be employed. The anvil connector 28 at the distal end of the mandrel 4 is a thread that provides a shear force of approximately twice the shear force of the tool thread 30 at the proximate end of the mandrel 4. The cylindrical body portion 48 of the mandrel 4 is sized to slidably engage the other components discussed with respect to FIG. 2. In FIG. 3, a central bore 38 extends from the proximate end to the distal end of the mandrel 4, which enables fluids to pass through the plug, as is necessary during times when the plug is not set, as will be appreciated by those skilled in the art. A ball seal rim 36 is provided along the central bore 38. Once the plug is set in the tubular (not shown), a ball (item 37 shown in phantom line) is dropped into the tubular (not shown) and comes to rest on the ball seal rim 36. This implements a check valve function where the plug is sealed from higher pressure above, but relieves for a higher pressure below the plug. The utility of this action will be appreciated by those skilled in the art. In other embodiments, the ball 37 may be set into the mandrel 4 at the time of manufacture, and a cross pin (not shown) is inserted through the mandrel 4 to retain the ball 37 in place. Such an arrangement is referred to as a “caged ball” by those skilled in the art.

At the proximate end of the mandrel 4 has an increased diameter portion 46 to provide material strength to accommodate the tool thread 30 forces and also to act as a travel limit for when the setting sleeve (not shown) engages the setting thread 26. The proximate face of the mandrel 4 presents the first clutch face 12. The first clutch face 12 is a ratchet clutch in this illustrative embodiment, and comprises a series of asymmetrical teeth, each having an inclined surface 50 meeting a drive face 54 at an acute angle 50. Other clutch face designs will be discussed hereinafter, and include at least crown gear configurations and Hirth coupler configurations. In the illustrative embodiment, the drive face 54 is machined with a drill bit or circular mill tool to simplify and reduce cost of fabrication.

Reference is directed to FIG. 5 and FIG. 6, which are a perspective view drawing and a side view drawing, respectively, of a setting sleeve 6 for a tubular plug according to an illustrative embodiment of the present invention. The setting sleeve 6 is a ring fabricated from fiber reinforced plastic that has a threaded central bore 62, which engages the setting threads (not shown) of the mandrel (not shown). The shear force for this threaded connection may be controlled by adjusting the design of either of these threads 62, or the setting threads on the mandrel (not shown). The setting sleeve 6 includes a substantially planar lower, or distal, face 56 that engages the slip ring (not shown). A planar upper, or proximate face, 60 engages a setting tool (not shown) at the time the plug is set in the tubular. Plural recesses 58 are formed in the substantially planar distal face 56 of the setting sleeve 6. These recesses 58 engage the slip pieces (not shown) at the time the plug is drilled out so as to prevent the setting sleeve 6 from rotating with the drill bit. This action enables the drill bit to more quickly drill through the setting sleeve 6. In other embodiments, the widths of the recesses 58 are selected to match the widths of the slip elements to facilitate the prevention of rotation therebetween. The recesses may be formed radially or at other angles depending on the configuration of the slip pieces. The object is to form the recesses such that they readily engage the slip pieces to prevent rotation therebetween.

Reference is directed to FIG. 7 and FIG. 8, which are a perspective view drawing and a section view drawing, respectively, of a cone 18, 20 for a tubular plug according to an illustrative embodiment of the present invention. In the illustrative embodiment, the cones 18, 20 are fabricated from fiber reinforced plastic material. There are two cones 18, 20 in the illustrative embodiment, one on each disposed on either side of the elastomeric seal member (not shown). The larger diameter planar end 68 engages the sealing member (not shown), and the conical surface 70 engages an interior conical surface of the slips (not shown). The conical surface 70 is configured as a frustum of a cone, and terminates at a smaller planar surface 66. A central bore 64 is formed through the cones 18, 22, and is sized to slide on the mandrel cylindrical body (not shown).

Reference is directed to FIG. 9, FIG. 10, and FIG. 11, which are a side view drawing, an end view drawing, and a section view drawing, respectively, of a slip ring 22, 24 for a tubular plug according to an illustrative embodiment of the present invention. In the illustrative embodiment, the slips 22, 24 are fabricated as a ring-like structure that is formed or drilled with plural holes 74, as illustrated, which define weakened sections that readily fracture into plural individual slip elements 76 at the time the plug (not shown) is set into the tubular (not shown). The slip ring 22, 24 has a planar drive surface 86, which engages either the anvil (not shown) or the setting sleeve (not shown), depending on whether the slip is positioned above of below the sealing element (not shown). The exterior surface 72 of each individual slip element 76 is formed in a saw tooth pattern 72 so as to present edges that engage the tubular interior surface (not shown) when the slip is set, thereby locking the plug (not shown) in place in the tubular (not shown). The interior surface 78 is formed as a frustum of a cone, which shape corresponds to the exterior shape of the cones (not shown). The conical engagement provides an inclined surface that translates the axial force applied by the setting tool (not shown) into radial forces to fracture and set the slip rings 22, 24. In the illustrative embodiment, the slip rings 22, 24 are fabricated from cast ductile iron that is case hardened using a ferritic nitrocarburizing process. Other types of iron and other types of case hardening techniques could also be employed. Also, composite slip elements could be employed.

Reference is directed to FIG. 12 FIG. 13, FIG. 14, and FIG. 15, which are a perspective view drawing, a side view drawing, an end view drawing, and a section view drawing, respectively, of an anvil 8 for a tubular plug according to an illustrative embodiment of the present invention. The anvil 8 of the illustrative embodiment serves as the fulcrum against which the setting tool (not shown) works to set the plug (not shown). The anvil 8 is connected to the distal end of the mandrel (not shown) using a connection means, which is a threaded internal bore 96 in the illustrative embodiment. The anvil could also be pinned, cemented, interlocked, bonded, or otherwise connected, as will be appreciated by those skilled in the art. The upper, or proximate, face 102 of the anvil 8 engages the lower slip ring (not shown) in the assembled plug (not shown). The lower, or distal, face of the anvil 8 presents the second clutch face 14. The second clutch face 14 is a ratchet clutch in this illustrative embodiment, and comprises a series of asymmetrical teeth, each having an inclined surface 90 meeting a drive face 94 at an acute angle 92. Other clutch face designs can be employed, and include at least crown gear configurations and Hirth coupler configurations. The salient feature of the clutch face 14 is that it engages another object to limit rotation therebetween, particularly including a corresponding clutch face disposed on the proximate end of the mandrel (not shown). It will be understood by those skilled in the art that oilfield down hole tools rotate clockwise when viewed from above. In the illustrative embodiment, the drive face 94 is machined with a drill bit or circular mill tool that is aligned radially to the anvil 8. This design simplifies machining operations and reduces cost of the part.

The outer body of the anvil 8 includes a frustoconical surface 100 that has plural helical flutes 34 formed onto its surface. The flutes provide a channel to clear material and debris from the tubular (not shown) while the plug and anvil are drilled out. It is a function likened to a fluted drill bit. In addition, the clutch face 14 also functions as a drill bit while the plug and anvil are being drilled out. In the illustrative embodiment, the plural helical flutes 34 intersect with the driving face 94 and vertices 92 so as to function cooperatively with the flutes 34 for the drilling function. In addition, the flutes 34 can be machined with the same tool that drills the drive faces 94, to thereby further simplify machining and reduce fabrication costs. The threaded central bore 96 of the anvil 8 has a bore extension 98 to allow fluids to flow through the anvil 8. In the illustrative embodiment, the anvil is fabricated from fiber reinforced plastic, which is readily drillable using a convention oilfield tri-cone drill bit. Not also that the distal face 14 of the anvil 8 may receive a ball 37 (shown in phantom line), which may be pushed upward by higher formation pressure below a given plug, and come to rest against the distal face 14. This ball might come from another plug that has already been set below. A salient feature of this arrangement is that the distal face 14 will not form a pressure seal with the ball 37 by virtue of the clutch face angular cuts, but rather, will allow the higher formation pressure from below to be relieved as well fluids pass upwardly around the ball 37 and though the bore extension passage 98 in the anvil 8. The advantages of this arrangement will be appreciated by those skilled in the art.

Reference is directed to FIG. 16, which is a side view drawing of an anvil for a tubular plug according to an illustrative embodiment of the present invention. This embodiment illustrates an alternative anvil 104 configuration where a saw tooth clutch face is employed. This embodiment includes a series of asymmetrical teeth, each having an inclined surface 106 meeting a flat drive face 108 at an acute angle 110. This design is more in line with a convention ratchet. FIG. 17 is a partial side view of the proximate end of a mandrel 112 that corresponds to the anvil 104 in FIG. 16. In FIG. 17, the mandrel 112 includes a series of asymmetrical teeth, each having an inclined surface 116 meeting a flat drive face 118 at an acute angle 114.

Reference is directed to FIG. 18, which is a section view drawing of a well casing 120 with plural tubular plugs 126, 128, and 130 therein according to an illustrative embodiment of the present invention. This figure presents an operation where plural plugs 126, 128, 130 are being drilled out using a tri-cone bit 124 on a tool string 122. In this example, the top plug 126 has been drilled to where the slips have already released and the top plug 126 has fallen, or has been pushed, down to the middle plug 128. The bottom plug, 130, is further below, and has not yet been affected by the drill-out operation. Note that the clutch faces 132 of the upper and middle plugs 126 and 128 have engaged such that the fixed position of the middle plug 128 prevents the upper plug 126 from rotating with the tri-cone drill 124. In this manner, the tri-cone drill 124 more efficiently grinds the upper plug 126.

Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.

It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 

1. A drillable plug assembly, which is settable using a tool, for isolating differential pressures in a tubular, comprising: a mandrel, formed of composite material, having a cylindrical body with a first clutch face formed on a proximate end thereof, and having a tool thread formed within a central bore thereof that shears at a first predetermined axial force, and having a setting thread disposed about the exterior thereof, and having an anvil connector disposed at a distal end thereof; a setting sleeve disposed about said cylindrical body, and threadably engaged with said setting thread to shear at a second predetermined axial force applied thereagainst; an anvil, formed of composite material, connected to said anvil connector, having a second clutch face formed on a distal end thereof; a seal assembly, including an elastomeric seal and a slip for sealably and fixedly engaging the tubular upon setting by the tool, disposed about said cylindrical body between said setting sleeve and said anvil, and wherein said first ratchet clutch face and said second ratchet clutch face are configured to cooperatively engage one another and prevent rotation therebetween, to thereby enable plural drillable plug assemblies in the tubular to resist rotation therebetween as they are drilled out of the tubular.
 2. The assembly of claim 1, and wherein: said composite material is fiber reinforced plastic.
 3. The assembly of claim 1, and wherein: said composite material comprises epoxy resin reinforced with filament wound fiber selected from glass, aromatic polyamide, and carbon.
 4. The assembly of claim 1, and wherein: said first clutch face and said second clutch face are configured as interlocking crown gears.
 5. The assembly of claim 1, and wherein: said first clutch face and said second clutch face are configured as mating Hirth coupling faces.
 6. The assembly of claim 1, and wherein: said first clutch face and said second clutch face are configured as ratchet clutches.
 7. The assembly of claim 6, and wherein: said ratchet clutches comprise a series of asymmetrical teeth, each having an inclined surface meeting a drive face at an acute angle.
 8. The assembly of claim 1, and wherein: said second predetermined axial force is greater than said first predetermined axial force, to thereby cause said setting sleeve to shear from said mandrel prior to the tool shearing from said tool thread.
 9. The assembly of claim 1, and wherein: said setting sleeve includes a sleeve thread the that shears at said second predetermined axial force.
 10. The assembly of claim 1, and wherein: said anvil connector is an anvil thread disposed adjacent to said distal end on said cylindrical body, and wherein said anvil threadably engages said anvil thread.
 11. The assembly of claim 1, and wherein said seal assembly slidably engages said tubular body between said setting sleeve and said anvil, and further comprising: a first setting cone and a second setting cone disposed on either side of said elastomeric seal, each having a conical portion extending away from said elastomeric seal, and wherein said slip comprises a pair of iron slips having conical interior surfaces that cooperatively engage said pair of setting cones such that they are driven radially into the tubular in response to axial force that exceeds said first predetermined axial force, to thereby compress said elastomeric seal, and locate and set the drillable plug assembly in the tubular.
 12. The assembly of claim 11, and wherein: said setting sleeve and said first and second cone are formed of composite material, and wherein said pair of iron slips are fabricated from ductile iron.
 13. The assembly of claim 1, and wherein: said anvil is configured with a frustoconical surface extending from said second clutch face, and wherein plural helical flutes are formed upon said frustoconical surface, to thereby facilitate removable of material within the tubular as the drillable plug assembly is drilled.
 14. The assembly of claim 13, and wherein: said second clutch face on said distal end of said anvil comprises and plural inclined surfaces and plural drive surfaces that meet and at plural acute angle vertexes, and wherein said plural helical flutes each intersect one of said plural acute angle vertexes.
 15. The assembly of claim 1, and wherein said slip includes a proximate planar surface that is engaged by a distal planar surface of said setting sleeve, to thereby transmit axial force from said setting sleeve to said slip as the drillable plug assembly is set by the setting tool.
 16. The assembly of claim 15, and wherein: said slip includes plural sections that fracture into slip section pieces as the drillable plug assembly is set by the setting tool, and wherein said distal planar surface of said setting sleeve includes plural recesses formed therein, which are configured to engage said slip section pieces to thereby prevent rotation therebetween as the drillable plug assembly is drilled out of the tubular.
 17. The assembly of claim 16, and wherein: said plural recesses are formed as radial grooves in said distal planar surface of said setting sleeve.
 18. The assembly of claim 16, and wherein: said plural recesses have a width selected to correspond to the size of said slip section pieces to thereby facilitate engagement therebetween.
 19. The assembly of claim, and wherein: said second clutch face of said anvil is configured to retain a ball forced against it by a pressure differential, but without sealing thereagainst, to thereby relieve said pressure differential by flow of fluids between said ball and said second clutch face.
 20. The assembly of claim 1, and wherein: said central bore of said anvil extents continuous from said proximate end to said distal end, and comprises a ball seat disposed there along for retaining a ball, to thereby provide a passage through which fluids may pass and a means for sealing said central bore using a ball placed against said ball seat.
 21. The assembly of claim 1, and wherein: said slip is fabricated from iron which has been surface hardened by ferritic nitrocarburization.
 22. The assembly of claim 1, and wherein: said slip is fabricated as a ring that has plural holes formed therethrough to facilitate fracturing of said slip when set by the tool, and wherein said slip has been surface hardened by ferritic nitrocarburization.
 23. The assembly of claim 11, and wherein: said pair of iron slips have been surface hardened by ferritic nitrocarburization.
 24. The assembly of claim 11, and wherein: said pair of slip are fabricated as rings that have plural holes formed therethrough to facilitate fracturing of said slips when set by the tool, and wherein said slips have been surface hardened by ferritic nitrocarburization. 